As one of wireless transmission methods excellent in multipath resistance characteristic, an OFDM (Orthogonal Frequency Division Multiplexing) method is known in which intersymbol interference caused by multipath is removed by dividing a total bandwidth into a plurality of subcarriers, inserting guard intervals, and removing the guard intervals on a receiving side.
As one of techniques for frequency-multiplexing a plurality of users in the OFDM method, there is known a method of using a different frequency hopping pattern for each user (for example, see U.S. Pat. No. 5,548,582).
Hereinafter, referring to FIGS. 1 and 2, description will be made of a communication system using frequency hopping.
In a transmitter 6-1, an encoder 61 encodes a transmission sequence STS1 and produces a coded sequence SCS. An interleaver 62 interleaves the coded sequence SCS and produces an interleaved sequence SIS. A symbol mapping portion 63 maps the interleaved sequence SIS into modulated symbols and produces a transmission symbol sequence STSY.
A hopping pattern generating portion 64 produces a hopping pattern SHP1 unique to the transmitter. According to the hopping pattern SHP1, a subcarrier assigning portion 65 assigns the transmission symbol sequence STSY to subcarriers 1 to R and produces frequency hopping signals SFH1 to SFHR.
A high-speed inverse Fourier transformer 66 carries out high-speed inverse Fourier transform upon the frequency hopping signals SFH1 to SFHR and produces an IFFT signal SIFFT. A guard interval adding portion 67 adds guard intervals to the IFFT signal SIFFT and delivers a transmission signal STSX1 via an antenna 68. Transmitters 6-2 to 6-K are operated in the manner similar to the above-mentioned transmitter 6-1 and, in response to transmission sequences STS2 to STSK, produce transmission signals STSX2 to STSXK, respectively.
In a receiver 7, a guard interval removing portion 72 removes the guard intervals from a reception signal SRX supplied to an antenna 71 and produces a FFT input signal SFFTI. A high-speed Fourier transformer 73 performs high-speed Fourier transform upon the FFT input signal SFFTI and produces FFT signals SFFT1 to SFFTR.
A hopping pattern generating portion 74 produces unique hopping patterns SHP1 to SHPK corresponding to the transmitters 6-1 to 6-K, respectively. A subcarrier extracting portion 75 extracts, from the FFT signals SFFT1 to SFFTR, components corresponding to the hopping patterns SHP1 to SPHK, respectively, and outputs the components as demodulated sequences SDMS1 to SDMSK.
Deinterleavers 76-1 to 76-K deinterleave the demodulated sequences SDMS1 to SDMSK and produce deinterleaved sequences SDIS1 to SDISK, respectively. Decoders 77-1 to 77-K decode the deinterleaved sequences SDIS1 to SDISK and produce decoded sequences SDCS1 to SDCSK, respectively.
For example, transmission sequences correspond to different users, respectively. It is assumed that the transmission sequences are equal in number to four and frequency channels are equal in number to four. In this case, when users #1 to #4 carry out frequency hopping by the use of hopping patterns (#1, #3, #2, #4), (#2, #1, #4, #3), (#3, #4, #1, #2), (#4, #2, #3, #1), respectively, all of the users are orthogonal on a frequency axis at a same time instant, as illustrated in FIG. 3. All of the frequency channels are completely used by the respective users so that a frequency diversity effect is achieved.
On the other hand, MIMO (Multiple-Input Multiple-Output) using a plurality of antennas for transmission and reception is known as a method capable of improving a frequency usability by parallel transmission utilizing independency of propagation paths (for example, see Non-Patent Document: “Maximum Likelihood Decoding in a Space Division Multiplexing System” in IEEE VTC 2000 Spring Proceedings (R. van Nee et al, May 2000, pages 6 to 10).
Referring to FIGS. 4 and 5, description will be made about a communication system based on OFDM with frequency hopping applied thereto, in case where MIMO using two transmission antennas and two reception antennas is further applied.
In a transmitter 8-1, an encoder 81 encodes a transmission sequence STS1 and produces a coded sequence SCS. An interleaver 82 interleaves the coded sequence SCS and produces an interleaved sequence SIS. A serial/parallel converter 83 performs serial/parallel conversion upon the interleaved sequence SIS and produces serial/parallel signals SSP1 and SSP2.
Symbol mapping portions 84 and 85 map the serial/parallel signals SSP1 and SSP2 into modulated symbols and produce transmission symbol sequences STSY1 and STSY2, respectively. A hopping pattern generating portion 84 produces a hopping pattern SHP1 unique to the transmitter. According to the hopping pattern SHP1, subcarrier assigning portions 87 and 88 assign the transmission symbol sequences STSY1 and STSY2 to subcarriers 1 to R and produce frequency hopping signals SFH11 to SFH1R and SFH21 to SFH2R, respectively.
High-speed inverse Fourier transformers 89 and 90 carry out high-speed inverse Fourier transform upon the frequency hopping signals SFH11 to SFH1R and SFH21 to SFH2R and produce IFFT signals SIFFT1 and SIFFT2, respectively. Guard interval adding portions 91 and 92 add guard intervals to the IFFT signals SIFFT1 and SIFFT2 and produce transmission signals STSX11 and STSX12. Transmitters 8-2 to 8-K are operated in the manner similar to the above-mentioned transmitter 8-1 and, in response to transmission sequences STS2 to STSK, produce transmission signals STSX21, STSX22, . . . , STSXK1, STSXK2, respectively.
In a receiver 10, guard interval removing portions 103 and 104 remove the guard intervals from reception signals SRX1 and SRX2 supplied to antennas 101 and 102 and produce FFT input signals SFFTI1 and SFFTI2, respectively. High-speed Fourier transformers 105 and 106 perform high-speed Fourier transform upon the FFT input signals SFFTI1 and SFFTI2 and produce FFT signals SFFT11 to SFFT1R and SFFT21 to SFFT2R, respectively.
A hopping pattern generating portion 107 produces unique hopping patterns SHP1 to SHPK corresponding to the transmitters 8-1 to 8-K, respectively. Subcarrier extracting portions 108 and 109 extract, from the FFT signals SFFT11 to SFFT1R and SFFT21 to SFFT2R, components corresponding to the hopping patterns SHP1 to SPHK, respectively, and output the components as extracted sequences SEXT11 to SEXT1R and SEXT21 to SEXT2R.
A MIMO demodulating portion 110 combines and decomposes the extracted sequences SEXT11 to SEXT1R and SEXT21 to SEXT2R and produces partial demodulated sequences SPDM11, SPDM12, SPDM21, SPDM22, . . . , SPDMK1, SPDMK2.
Parallel/serial converters 111-1 to 111-K carry out parallel/serial conversion upon the partial demodulated sequences SPDM11, SPDM12, SPDM21, SPDM22, . . . , SPDMK1, SPDMK2 and produce demodulated sequences SDMS1 to SDMSK.
Deinterleavers 112-1 to 112-K deinterleave the demodulated sequences SDMS1 to SDMSK and produce deinterleaved sequences SDIS1 to SDISK, respectively. Decoders 113-1 to 113-K decode the deinterleaved sequences SDIS1 to SDISK and produce decoded sequences SDCS1 to SDCSK, respectively.
However, in the above-mentioned methods of applying frequency hopping and MIMO to the conventional OFDM, a distance between the transmission antennas can not sufficiently be large. If correlation between propagation paths is large, a diversity effect is small. In addition, signal separation at the MIMO demodulating portion is difficult, resulting in serious degradation in reception characteristics.